1. Field of the Invention
The present invention relates to a programmable read only semiconductor memory device (PROM), and more particularly, to a reading circuit for reading an electronic signature which is a type of read only memory (ROM) data relating to an attribute of the PROM.
2. Description of the Prior Art
Recently, the JEDEC (Joint Electron Device Engineering Council), which is a subordinate facility of the U.S. Electronic Industrial Association (EIA), proposed the use of so-called electronic signatures. That is, by previously writing the electronic signatures as one kind of ROM data such as manufacturers' codes or programming algorithm numbers into an erasable and programmable read only semiconductor memory device (EPROM), during its manufacture, these data can be read by applying a specific signal to a specific terminal of the EPROM. The contents of the data construction of the electronic signatures such as the manufacturers' codes are described later in more detail with reference to FIGS. 4 and 5.
Generally, in an EPROM, in order to improve the production yield of chips, a redundancy structure is employed so that a redundant circuit can replace a faulty circuit. Such redundancy techniques are disclosed in, for example, ISSCC 81/Wednesday, Feb. 18, 1981/BALL ROOMS A-B/3:15 P.M., pp 80-81, and ISSCC 82/Thursday, Feb. 11, 1982/CONTINENTAL BALL ROOMS 1-4/10:45 A.M., pp 112-113.
In an EPROM having a redundancy structure, it is necessary to correctly read out the contents of the data of the electronic signature corresponding to the replaced circuit portion when the redundancy circuit is used.